By Tom Symonds BBC Transport correspondent

It is a decision the government has been putting off for years - but where to build Britain's next major airport runway is a crucial one.

The airline industry is not mincing its words.

The consensus of opinion is nothing less than three new runways will do - if possible at Heathrow, Gatwick and Stansted.

	:	UK airport expansion
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Executives point to the government's own predictions of the growth in demand by 2020.

They suggest the number of air passengers will probably double, and could triple.

Whichever options are chosen, expect long and tortuous battles over planning

In the post-war years commercial flying used to be the preserve of the rich.

Then prices came down and it became a realistic way for tourists to travel, once a year, to their summer holiday, and business people to meetings.

Convenience flying

Now ticket prices are dirt cheap if you book carefully with budget airlines.

And we are using aircraft like our cars, for quick trips away for the weekend.

But it is not just the growth of aviation that is fuelling demands for extra runways.

Virgin's Sir Richard Branson says he has to buy bigger and bigger aircraft because there are so few available slots to land at the major London airports.

Each slot is worth so much to Virgin, it has to try and put as many passengers through as possible.

Sir Richard is calling for three new runways in southern England.

Growth 'over-estimated'

But anti-aviation campaigners believe the government has overestimated the need for more runways and terminals.

They say the predictions of growth are based on the experience of the 1990s, when the budget airlines were in their ambitious infancy.

The central decision the government has to take is whether to hold back the demand for air travel, meet some of the need, or meet all of it.

Airlines are battling for airport slots

Other countries - the Netherlands and France in particular - have chosen to invest for growth.

Schipol airport in Amsterdam and Paris Charles de Gaulle have both seen expansion programmes.

The government is offering numerous options for the future - though senior ministers insist no big decisions have been made, this is a genuine consultation.

Stansted 'ripe'

But what campaigners are calling the hit-list will centre on the expansion of existing airports, rather than building new ones.

Stansted is seen as ripe for growth.

It is situated in a more sparsely populated area than Heathrow or Gatwick, and has a rail link to London.

Stansted is also thought to have a smaller environmental lobby attached to it than other airports.

Heathrow is also on the list.

With a new terminal - the airport's fifth - shortly to begin construction, airlines want extra runway space to ease the pressure on the existing two runways.

But here there would be an almighty battle with residents and campaigners.

There is also the small matter of a legally-binding cap on flights, a government condition for agreeing to Terminal Five.

Regional option

Away from the south-east, ministers will have to decide if it's possible to turn one of the regional airports into an international hub.

Birmingham, Glasgow and Edinburgh are the favourites.

But airlines are not so keen.

The real money is to be made flying to London.

And then there is Cliffe - an area of the North Kent Thames Estuary thought to be one of the few places the transport consultants could find where an airport could be built.

Cliffe Marsh: The next Heathrow?

It is close to London - but the site is of environmental importance.

The main problem for the airline industry is building an airport means creating a brand name, to rival Heathrow or Gatwick.

And London Cliffe Airport does not really cut it.

Whichever options are chosen, expect long and tortuous battles over planning.

Perhaps they will not be as long as the row over Heathrow's Terminal Five, the subject of Britain's longest planning inquiry.

But this is a highly contentious issue.

The government's decisions will change the lives of those affected.